Add README for functionality outline.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

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+iltest is a test framework for InterLeaved TESTing. It was originally
+written for X.Org X Server testing.
+
+The principle: A process (the "testee") need to be tested by other processes
+(the "tester(s)").  The testers report on whether the tests were successful.
+
+A test has three stages:
+- pre: preparation stage
+- run: actual testing stage
+- post: cleanup stage
+
+In iltest, the order of calls is thus:
+
+ Time           Testee          Tester 
+  |              pre()
+  |                              pre()
+  |              run()
+  |                              run()
+  |              post()
+  V                              post()
+
+Thus, in the case of the X server testing, usually this happens:
+    testee.pre:  back up various files, ensure system integrity, etc.
+    tester.pre:  set up configuration files, add virtual devices, etc.
+    testee.run:  run the X server
+    tester.run:  run tests
+    testee.post: restore files
+    tester.post: gather test results
+
+iltest itself does not do much. It simply provides the interfaces for the
+test suites (which contain most of the knowledge) and a few utility
+functions.